This invention relates generally to a compact, high-efficiency, electrically small loop antennas for use in both transmitters and receivers of portable communication devices.
The physical size of modem compact communication devices (such as radio tags, personal communicators and pagers) often are dictated by the size of the antenna needed to make them function effectively. To avoid devices that are too large, pagers have made use of electrically small rectangular loop antennas as receive-only antennas with the maximum dimension of any antenna elements that constitute the antenna on the order of one-tenth of a wavelength or less of the receiving frequency. However, these small antennas tend to be inefficient as a result of their very low radiation resistance and comparatively high resistive loss. Likewise, as a result of their high inductive reactance (or Q) they tend to be sensitive to their physical environment. These small antennas have been known to cause parasitic oscillations in attached radio frequency (RF) circuitry. Finally, because of their low efficiency, these small antennas are inadequate as transmitter antennas.
To overcome the disadvantages of electrically small loop antennas, there is continuing need for antennas small in physical dimension (each element less than one-tenth of a wavelength, for example); having relatively high efficiency; capable of being placed in close proximity to associated electronic circuits without adversely affecting performance; capable of being used effectively for both transmitting and receiving; relatively insensitive to orientation and surroundings; easy to manufacture using standard, low-cost components; and capable of having radiation patterns altered to support different applications.
There is a need for antennas in general and in particular for efficient, dual-polarization, and three-dimensionally omnidirectional antennas that operate at VHF or UHF frequencies.
Such antennas are useful for general telecommunications applications. A particular need for such antennas exists in electronic inventory and tracking systems as an interrogator of radio tags attached to various remotely located items such as boxes or vehicles within a given area such as a warehouse or a parking lot. In such an application, a need exists for a relatively compact, structurally robust antenna that can satisfy the following conditions:
(1) Is tunable at high frequencies (specifically 315 and 433 MHz). PA1 (2) Operates efficiently enough to communicate with other antennas as far away as three-hundred feet while meeting the FCC limitations on maximum radiated power. PA1 (3) Is capable of communicating with antennas with unknown orientations and hence unknown polarization responses. PA1 (4) Where an array of two antenna elements is employed, minimal coupling between the antenna elements so that there is minimal signal distortion passed on to the receiver circuitry.
The term "omnidirectional" as used to describe antenna performance had various meanings in the literature. The term "omnidirectional" is often used when the radiation pattern of the antenna is constant in a single plane and usually only refers to the radiation pattern for a single polarization. The typical examples of this class of omnidirectional antennas are the short dipole and the small loop antenna. For example, U.S. Pat. Nos. 3,560,983 and 4,479,127 describe electrically small loop antennas with this type of omnidirectionality. However, that type of two-dimensional, single-polarization omnidirectionality is not sufficient for many purposes. When the antennas are not coplanar and the orientations of the other antennas are unknown, it is necessary to have a broader type of omnidirectionality.
Typically an army of two or more antenna elements with complementary polarization responses and complementary radiation patterns operating simultaneously gives greater omnidirectionality. For example, U.S. Pat. No. 4,814,777 describes an array of vertical monopole antennas and horizontal dipole antennas arranged on alternating coplanar and concentric circles. U.S. Pat. No. 3,945,013 describes an array consisting of a vertical monopole antenna and a slot antenna sensitive to horizontal polarization. In using antennas of these types it has been found first, that to achieve the required efficiency, the monopoles were too long to be structurally robust, and second, that to achieve the desired gain for a slot antenna resulted in low efficiency due to dielectric losses in the plastic material filling the slot even though such material provides greater structural robustness. Hence these types of designs have been found unsatisfactory.
U.S. Pat. Nos. 3,440,542 and 3,721,989 describe crossed loop antenna arrays consisting of multiple windings around ferrite cores. These antennas operate at 535-1650 kHz and 10-14 kHz respectively. These antennas are more compact and structurally more robust than antennas that use monopoles. Although those antennas are described as "omnidirectional", the omnidirectionality is for a single polarization, the vertical polarization, in the plane containing the ferrite cores. In other words, they have the same type of limited omnidirectionality mentioned above. In the plane of the ferrite cores (the omnidirectionality for the vertical polarization) the horizontal polarization radiation pattern has a null. Another shortcoming is that these antennas are inefficient for use at UHF frequencies. These antennas have a very large inductance due to the large number of turns of wire and to the high permeability of the ferrite cores. Tuning these antennas at UHF frequencies requires an impractically small capacitance (about 0.3-0.6 milli-pico-farads). Also, although not explicitly discussed these antennas are very inefficient in that they have high loss resistance relative to their radiation resistance and hence have very low gain. Typical gains for small loop antennas are on the order of about -20 dB. Furthermore, the use of ferrite cores at UHF frequencies would increase the loss resistance and hence decrease the efficiency prohibitively.
In the above-identified application entitled EFFICIENT ELECTRICALLY SMALL LOOP ANTENNA WITH A PLANAR BASE ELEMENT, an electrically small rectangular loop antenna is mounted on a rectangular, metal base plate. In that application, the base plate was planar base element that formed part of the radiating system and also acted as a shield between the circuitry and the radiator. That application included a new design for a capacitive matching network contained in windows in the metal plate. In that application, frequencies were used such that the overall dimensions of the radiator were of the order of .lambda./10 where .lambda. is the wavelength of the radiation. Thus, the radiation pattern tended to have the limited single polarization, two-dimensional omnidirectionality mentioned above.